Optical lenses are used in ophthalmic devices such as eyeglasses and contact lenses and in precision instruments such as cameras, telescopes, microscopes, and range finders. These lenses are typically made by imparting a specific curvature on a first side of a transparent material such as mineral glass or plastic, and a different curvature on the opposite side of the material. By creating a curve on the second side of the lens that is different than the curve on the first side of the lens, light can be focused to a desired point.
The process of producing a lens generally begins by first grinding or otherwise machining a glass or plastic blank to achieve the approximate curvature or curvatures desired. The grinding process creates surface roughness on the surface of the lens, which tends to undesirably scatter light passing to or from the lens. To reduce this surface roughness, the lens is polished to obtain a smoother surface. In addition, polishing can provide a more precise curvature to the lens surface allowing the light exiting the lens to be more accurately focused.
Blanks used for eyeglasses typically are made by injection molding or casting a thermosetting polymer such as diethylene glycol bis(allyl carbonate) (CR-39) or polycarbonate. These blanks typically measure between 70 and 80 mm in diameter and between 8 and 20 mm in thickness. The blank may also include a base curve that is close to the desired power of the lens. Once a blank with a base curve is formed, its back is ground to make a lens of the desired power (e.g. to match the eyeglasses prescription).
Most automated grinding machines have a cutter that is held stationary while rotating the lens and moving it along two axes with respect to the cutter. If the lens requires a curvature in addition to simple spherical and/or cylindrical cuts, the lens can be ground while tilted to produce an offset optical center (i.e. an induced prism). After the lens is ground, it is sanded and then polished. Polishing machines typically utilize a lap, which is an abrasive pad attached to a block having a matching, but reversed, curvature of the lens. The lap and lens are rubbed together to remove the surface roughness left by the grinding process and to make any final corrections to the curvature of the lens. This polishing method has the disadvantage of requiring a separate lap for each lens prescription. Thus, a typical lens processing facility will have hundreds if not thousands of different laps available to produce eyeglasses conforming to a wide range of prescription requirements.
More advanced polishing machines have recently been developed that utilize a pivoting head which carries a tool spindle to which a shaping tool, such as a polishing wheel, is attached. With respect to the lens, the rotating polishing wheel moves along a first horizontal axis (the “X-axis”) having a left-right orientation and along a second horizontal axis (the “Y-axis”) having a front-rear orientation. In addition, the spindle upon which the wheel is mounted moves vertically along a “Z-axis”. The spindle also moves in a circular direction about a “C-axis”. These advanced polishing machines typically utilizing a positional feed-back system to control the movement of the wheel.
Polishing wheels have a fine abrasive surface that can reduce the surface roughness of a lens when the abrasive surface contacts and moves across the surface of the lens. The surface of the wheel is typically curved in order to follow the curved contour of the lens surface. Thus, polishing wheels typically are of a cylindrical or spherical shape.
Generally, these polishing wheels have an axis and corresponding axial cavity for receiving a rotatable motor-driven spindle. The contact surface of the wheel is symmetrical with respect to this axis in order to allow for continuous contact between the wheel and lens while the wheel or lens is rotating about an axis.
Conventional polishing wheels typically have a urethane skin that is cut from a flat sheet and glued onto a spherical natural rubber substrate that surrounds a spherical aluminum hub. The flat sheet is cut in such a way as to allow it to be folded to conform to the spherical shape of the substrate. However, this folding technique invariably results in a discontinuous surface and gaps in the skin tend to form at the junctions of the folds. These gaps are partially responsible for the limited life of the polishing tool because they can catch on the edge of the lens during the polishing process and begin to tear away from the rubber substrate. Over time, the outer skin can also begin to crack at the intersection of the gaps.
The urethane skins known in the art are also difficult to replace once they become worn. Removing a worn urethane skin from the rubber substrate and replacing it with a new one requires the use of toxic chemicals. The rubber substrate of known wheels also suffer the tendency of pulling away from their respective aluminum hub. In addition, it is often difficult to produce and maintain a rubber substrate and outer urethane shell that is concentric with the aluminum hub. Polishing wheels with substrates, outer shells, or both that are not concentric to the hub can impart low frequency waves onto the surface of the lens during the polishing process which, in turn, reduces the accuracy of the polishing operation.